Why IRQ8 infects the timer on mpc8541?

Hi, All
    I am working for a project based on mpc8541. Decrementer was ok
when external interrupt was disabled( in my board, IRQ8 connected to
other PCI device, EIVPR8 value was 0x8001000B ), but when i set the
value 0x0000100B to EIVPR to set IRQ8 enable, decrementer does not
work( IP will not go to 0x900 offset).

Who can give a hand?

Thanks

Regards,
cchaoyu